This invention relates to a waveform generator, a field deflection circuit comprising a waveform generator, a display apparatus comprising a field deflection circuit, and a method of generating a periodic waveform.
U.S. Pat. No. 4, 891,565 discloses a field deflection circuit including a sawtooth generator for generating an essentially sawtooth-shaped voltage. The circuit is provided with a storage element in which information is stored under the influence of applied clock pulses. Per field period, the number of times when the content of the storage element is changed is reduced to a predetermined number, e.g. because a number of clock pulses is omitted. The clock pulses applied to the sawtooth generator are then distributed as regularly as possible. More in detail, a pulse reduction circuit (which may be a rate multiplier) converts a number of input pulses (the clock pulses) occurring during a vertical scan period into a fixed number of output pulses. Integrating a fixed value to an output voltage at each output pulse, generates a waveform. In this way, a linear waveform is generated with a constant amplitude (the fixed number of output pulses multiplied by the fixed value) independent of the duration of the field scan period.
It is a drawback of this prior art that the waveform is limited to a linear sawtooth only.
It is an object of the invention to provide a simple waveform generator with a pulse reduction circuit, which waveform generator is able to provide an arbitrary waveform.
To this end, a first aspect of the invention provides a waveform generator as claimed in claim 1. A second aspect of the invention provides a field deflection circuit comprising a waveform generator as claimed in claim 5. A third aspect of the invention provides a display apparatus comprising a field deflection circuit as claimed in claim 7. A fourth aspect of the invention provides a method of generating a periodical waveform as claimed in claim 8. Advantageous embodiments are defined in the dependent claims.
The waveform generator according to the first aspect of the invention generates a non-linear waveform by introducing a correction circuit which varies the number, fixed in the prior art, supplied to the pulse reduction circuit. Consequently, the pulse reduction circuit supplies a number of desired output pulses which varies in time corresponding to the varying number. For example, when a lower number is inputted to the pulse reduction circuit during a sub-period of the repetition period (the repetition period may be the field scan-period), a corresponding lower number of output pulses will be generated. Consequently, a lower number of increment values will be summed or integrated during this sub-period and the waveform rises less steeply.
For example, a non-linear waveform is required in a field deflection circuit of a cathode ray tube. A linear field scan requires an S-corrected sawtooth-shaped field deflection current. Another example of a non-linear waveform is a field frequency parabola as may be used for an East-West correction or dynamic focussing.
The invention can be used to generate a field frequency waveform as in the prior art, but may also generate a line frequency waveform, or a waveform having any desired periodicity.
In the prior art, the pulse reduction circuit generates a linear sawtooth waveform. If another waveform is required, a complex analog multiplier is used to multiply the sawtooth by a correction waveform.
In an embodiment of the invention, the correction circuit comprises a calculator which receives a synchronizing signal indicating the repetition period. During the repetition period, the calculator supplies a series of predetermined numbers to the pulse reduction circuit. Values of the successive numbers correspond to differentiated values of the desired non-linear periodic waveform.
In an embodiment of the invention as claimed in claim 4, two small pulse reduction circuits instead of one large pulse reduction circuit are implemented. The first pulse reduction circuit generates a fixed number q (for example, 256) of pulses during the repetition period. In this way, the repetition period is divided into a fixed number of q sub-periods or sections. In each of the sections, the correction circuit supplies a predetermined number to the second pulse reduction circuit which will output this predetermined number of pulses during this section. The non-linear waveform is obtained when the correction circuit calculates different predetermined numbers for different sections. In this way, the repetition period is divided into a fixed number of sections independent of the duration of the repetition period. The correction circuit is simply synchronized with the output signal of the first pulse reduction circuit to supply a predetermined number (of desired output pulses to be supplied by the second pulse reduction circuit) for each section. It is not required to separately synchronize the correction circuit with the repetition period to allow generation of the correct predetermined numbers during the correct sections to obtain the correctly shaped non-linear waveform.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.